HARARE, Zimbabwe, Feb. 11 -- Zimbabwean opposition leader Morgan Tsvangirai was sworn in as prime minister Wednesday, bringing his party to the top echelons of power after 10 years of struggle by joining a coalition government with bitter rival President Robert Mugabe, who has ruled the nation for nearly three decades.

The inauguration marked what could be a fresh start for Zimbabwe, as Mugabe, who led the country to liberation from white rule in 1980, ceded significant power to an opponent for the first time. But Tsvangirai, while vowing to rescue Zimbabwe's collapsed economy and starving, cholera-stricken population, warned supporters that sharing power with Mugabe would be difficult.

"The most important issue is that we are opening a new chapter for our country. To my fellow compatriots, there can be no turning back," Tsvangirai told thousands of backers who gathered at a stadium here in the capital for an opposition rally after the swearing-in ceremony. "It is not a perfect agreement, but it is still a workable one."

The arrangement represents a remarkable achievement for Tsvangirai, 57, a former mineworker and union leader who survived beatings, jailings and assassination attempts as he sought to loosen Mugabe's control. On Wednesday, Mugabe administered the oath of office for Tsvangirai, shaking the hand of the man he has long denigrated as a lackey of the West.

Jubilant opposition backers at the rally gave Tsvangirai a hero's welcome. Several said the coalition government offered the best way to fix Zimbabwe's crisis and foreshadowed an end to Mugabe's sometimes brutal reign.

"He is like a cornered lion," said Patrick Chihota, 32, the owner of a printing company. "Now we don't fear Mugabe. We are free."

The sense of promise echoed scenes from five months ago, when Mugabe and Tsvangirai signed the power-sharing agreement. That followed elections in the spring, when Tsvangirai outpolled Mugabe in a first round but pulled out of a runoff, citing state-sponsored violence against his backers.

But tense negotiations ensued, hampered by disputes over cabinet slots and the opposition's doubts about Mugabe's intentions. Tsvangirai's party, the Movement for Democratic Change (MDC), agreed to join the government last month, after it became clear that leaders of other southern African nations would not join calls from the West for Mugabe to step down.

MDC officials, who refer to the unity government as a "transitional government," have said their key goals are to hold national elections within two years, repeal oppressive legislation and rewrite Zimbabwe's constitution. The party and a splinter opposition group will hold 16 of 31 ministries -- including education, health and finance -- that they believe can quickly address Zimbabwe's humanitarian crisis.

There have been signs of cooperation. Parliament, for example, passed legislation this week to give the parties joint oversight of Zimbabwe's security forces, which Mugabe has unleashed in the past to stifle dissent. But one of the MDC's key demands -- the release of about 30 opposition and civil society activists from prison -- has not been met, and two of the detainees' attorneys were arrested this week.

Many analysts and some skeptics within the opposition are wary. They say hard-line Mugabe loyalists, who have thrived on patronage amid economic chaos, might try to co-opt MDC members of government, harm them or simply frustrate them at every turn.

"We could wind up in a situation that's more or less the status quo, so the underlying reasons for the crisis will not go away at all, but this time it will be by a government that is legitimated," said Laurence Caromba of the Center for International Political Studies in Pretoria, South Africa.

Even if it works harmoniously, the new government faces daunting challenges. It must encourage investment and industry by stabilizing the economy, which is based almost entirely on foreign currency. It must pay civil servants, many of whom stopped working as their salaries lost value, and help feed the 7 million people the United Nations says are dependent on food aid.

To do so, the government hopes to persuade Western donors to flood Zimbabwe with funds for redevelopment, something many have pledged not to do without clear evidence of power-sharing. In Washington, State Department spokesman Robert A. Wood said Wednesday that the United States would "not even consider providing additional development or even easing sanctions until we see effective governance in the country."

But economists said Zimbabwe, once Africa's agricultural powerhouse, could reform its economy fairly quickly and attract foreign investment by reversing such policies as the rampant printing of now-worthless money and seizures of white-owned commercial farms.

"It's the policies that have destroyed the economy, not markets," said John Robertson, an independent economic consultant in Harare.

At the rally, Tsvangirai ticked off promises: an end to a "culture of impunity," the restoration of independent media, the removal of duties on imported food, and payment in foreign currency for all civil servants by the end of February. Though he did not say where the foreign money would come from, the pledges drew raucous applause.

Tsvangirai also called for reconciliation with "enemies." Nineteen years earlier to the day, he told the crowd, Nelson Mandela walked out of prison in South Africa and went on to peacefully negotiate a transition to democracy with the government that had jailed him for 27 years. Zimbabwe needs "national healing," too, Tsvangirai said.

"But at the same time, not forgetting what has happened, because tomorrow it can happen again," Tsvangirai said, jutting his finger toward the sky as his audience cheered.

After the rally, supporters professed optimism and seemed overjoyed to refer to "Prime Minister Tsvangirai." A group of men listed many problems -- health care, education, high passport fees -- but also spoke of a sudden feeling of freedom.

"We are free to talk!" said Takunda Mubwandari, 28, who said he would return to his job as a teacher if his salary, as promised by Tsvangirai, is paid in foreign currency.

But some voiced doubt. Felix Kumaka, who traveled 60 miles from his village for the rally, lifted his left pant leg to reveal a brace around his ankle. It was broken before last year's runoff election by rampaging Mugabe supporters, he said.

"We are not sure whether we can trust these ZANU-PF people. If you look at the way they treated us, I don't think they were doing all this violence so that they could share power with us," said Kumaka, 51, referring to Mugabe's Zimbabwe African National Union-Patriotic Front. "I think they are bitter."